you can deduct on Schedule A the real estate taxes that have been assessed and paid on all properties that you own, including any unimproved lots. (subject to any limits from 2018 -2025 on dedctiblity of schedule a taxes
instead of claiming a current deduction, you can elect to “capitalize” (add to the cost basis) any real estate taxes paid on unimproved and unproductive land (i.e., a vacant lot) held for investment. A statement must be attached to the original Form 1040 for each year that the election is made. IRS REG 1.266(c)(3)
If the taxpayer elects to capitalize an item or items under this section, such election shall be exercised by filing with the original return for the year for which the election is made a statement indicating the item or items (whether with respect to the same project or to different projects) which the taxpayer elects to treat as chargeable to capital account.
if you failed to include the election each year yo should have deducted the taxes. thus they would not add to basis to reduce gain. if you failed to included the election you may want to consult a tax pro as to options. worse case is that none of the taxes add to basis so you gain would be selling price less selling costs less original cost
